diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/ChunkManager.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/ChunkManager.java
index 91582b2f1a..c5a0c252ef 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/ChunkManager.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/ChunkManager.java
@@ -147,7 +147,7 @@ class ChunkManager {
 
   void adjustPadding() {
     if (paddingDivs != null) {
-      double h = host.getLineHeightPx();
+      double h = cmB.extras().lineHeightPx();
       for (Element div : paddingDivs) {
         int lines = div.getPropertyInt(DATA_LINES);
         div.getStyle().setHeight(lines * h, Unit.PX);
@@ -291,7 +291,9 @@ class ChunkManager {
     return new Runnable() {
       @Override
       public void run() {
-        int line = cm.hasActiveLine() ? cm.getLineNumber(cm.activeLine()) : 0;
+        int line = cm.extras().hasActiveLine()
+            ? cm.getLineNumber(cm.extras().activeLine())
+            : 0;
         int res = Collections.binarySearch(
                 chunks,
                 new DiffChunkInfo(cm.side(), line, 0, false),
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
index 8d8f117dc2..887978fdf3 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/CommentManager.java
@@ -91,8 +91,8 @@ class CommentManager {
         // It is only necessary to search one side to find a comment
         // on either side of the editor pair.
         SortedMap<Integer, CommentGroup> map = map(src.side());
-        int line = src.hasActiveLine()
-            ? src.getLineNumber(src.activeLine()) + 1
+        int line = src.extras().hasActiveLine()
+            ? src.getLineNumber(src.extras().activeLine()) + 1
             : 0;
         if (dir == Direction.NEXT) {
           map = map.tailMap(line + 1);
@@ -295,9 +295,9 @@ class CommentManager {
     return new Runnable() {
       @Override
       public void run() {
-        if (cm.hasActiveLine()) {
+        if (cm.extras().hasActiveLine()) {
           CommentGroup w = map(cm.side()).get(
-              cm.getLineNumber(cm.activeLine()) + 1);
+              cm.getLineNumber(cm.extras().activeLine()) + 1);
           if (w != null) {
             w.openCloseLast();
           }
@@ -310,9 +310,9 @@ class CommentManager {
     return new Runnable() {
       @Override
       public void run() {
-        if (cm.hasActiveLine()) {
+        if (cm.extras().hasActiveLine()) {
           CommentGroup w = map(cm.side()).get(
-              cm.getLineNumber(cm.activeLine()) + 1);
+              cm.getLineNumber(cm.extras().activeLine()) + 1);
           if (w != null) {
             w.openCloseAll();
           }
@@ -327,8 +327,8 @@ class CommentManager {
         @Override
         public void run() {
           String token = host.getToken();
-          if (cm.hasActiveLine()) {
-            LineHandle handle = cm.activeLine();
+          if (cm.extras().hasActiveLine()) {
+            LineHandle handle = cm.extras().activeLine();
             int line = cm.getLineNumber(handle) + 1;
             token += "@" + (cm.side() == DisplaySide.A ? "a" : "") + line;
           }
@@ -340,7 +340,7 @@ class CommentManager {
     return new Runnable() {
       @Override
       public void run() {
-        if (cm.hasActiveLine()) {
+        if (cm.extras().hasActiveLine()) {
           newDraft(cm);
         }
       }
@@ -348,7 +348,7 @@ class CommentManager {
   }
 
   private void newDraft(CodeMirror cm) {
-    int line = cm.getLineNumber(cm.activeLine()) + 1;
+    int line = cm.getLineNumber(cm.extras().activeLine()) + 1;
     if (cm.somethingSelected()) {
       FromTo fromTo = cm.getSelectedRange();
       Pos end = fromTo.to();
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
index 8bdc7477d1..10b2640e6a 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.java
@@ -48,14 +48,11 @@ class DiffTable extends Composite {
     String dark();
     String diff();
     String noIntraline();
-    String activeLine();
     String range();
     String rangeHighlight();
-    String showTabs();
     String showLineNumbers();
     String hideA();
     String hideB();
-    String columnMargin();
     String padding();
   }
 
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
index 1420d3b7e3..a0b84468f2 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DiffTable.ui.xml
@@ -18,12 +18,11 @@ limitations under the License.
     xmlns:g='urn:import:com.google.gwt.user.client.ui'
     xmlns:d='urn:import:com.google.gerrit.client.diff'>
   <ui:style type='com.google.gerrit.client.diff.DiffTable.DiffTableStyle'>
-    @external .CodeMirror, .CodeMirror-lines, .CodeMirror-selectedtext;
+    @external .CodeMirror, .CodeMirror-selectedtext;
     @external .CodeMirror-linenumber;
     @external .CodeMirror-overlayscroll-vertical, .CodeMirror-scroll;
     @external .CodeMirror-dialog-bottom;
     @external .cm-animate-fat-cursor, .CodeMirror-cursor;
-    @external .cm-searching, .cm-trailingspace, .cm-tab;
 
     .fullscreen {
       background-color: #f7f7f7;
@@ -39,13 +38,10 @@ limitations under the License.
       -ms-user-select: none;
     }
 
-    .difftable .CodeMirror-lines { padding: 0; }
     .difftable .CodeMirror pre {
-      padding: 0;
       overflow: hidden;
       border-right: 0;
       width: auto;
-      line-height: normal;
     }
 
     /* Preserve space for underscores. If this changes
@@ -106,23 +102,12 @@ limitations under the License.
       overflow-x: auto;
     }
 
-    .activeLine .CodeMirror-linenumber {
-      background-color: #bcf !important;
-      color: #000;
-    }
-
     .range {
       background-color: #ffd500 !important;
     }
     .rangeHighlight {
       background-color: #ffff00 !important;
     }
-    .cm-searching {
-      background-color: #ffa !important;
-    }
-    .cm-trailingspace {
-      background-color: red !important;
-    }
     .difftable .CodeMirror-selectedtext {
       background-color: inherit !important;
     }
@@ -154,24 +139,10 @@ limitations under the License.
       bottom: auto;
       left: auto;
     }
-    .showTabs .cm-tab:before {
-      position: absolute;
-      content: "\00bb";
-      color: #f00;
-    }
     .showLineNumbers .padding {
       margin-left: 21px;
       border-left: 2px solid #d64040;
     }
-    .columnMargin {
-      position: absolute;
-      top: 0;
-      bottom: 0;
-      width: 0;
-      border-right: 1px dashed #ffa500;
-      z-index: 2;
-      cursor: text;
-    }
 
     .diff_header {
       font-size: 12px;
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Scrollbar.css b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Scrollbar.css
index a5351151f4..383f2782cf 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Scrollbar.css
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Scrollbar.css
@@ -13,26 +13,6 @@
  * limitations under the License.
  */
 
-@external .CodeMirror;
-@external .CodeMirror-overlayscroll-horizontal;
-@external .CodeMirror-overlayscroll-vertical;
-
-.CodeMirror-overlayscroll-horizontal div {
-  min-width: 25px;
-}
-.CodeMirror-overlayscroll-vertical div {
-  min-height: 25px;
-}
-
-.CodeMirror .CodeMirror-overlayscroll-vertical {
-  z-index: inherit;
-}
-.CodeMirror .CodeMirror-overlayscroll-horizontal div,
-.CodeMirror .CodeMirror-overlayscroll-vertical div {
-  background-color: rgba(128, 128, 128, 0.50);
-  z-index: 8;
-}
-
 .comment, .draft, .insert, .delete, .edit {
   min-height: 5px;
   position: absolute;
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java
index 42b49d3121..2675235e57 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java
@@ -48,7 +48,6 @@ import com.google.gwt.core.client.Scheduler.RepeatingCommand;
 import com.google.gwt.core.client.Scheduler.ScheduledCommand;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.NativeEvent;
-import com.google.gwt.dom.client.Style;
 import com.google.gwt.event.dom.client.FocusEvent;
 import com.google.gwt.event.dom.client.FocusHandler;
 import com.google.gwt.event.dom.client.KeyCodes;
@@ -58,7 +57,6 @@ import com.google.gwt.event.logical.shared.ResizeHandler;
 import com.google.gwt.event.shared.HandlerRegistration;
 import com.google.gwt.uibinder.client.UiBinder;
 import com.google.gwt.uibinder.client.UiField;
-import com.google.gwt.user.client.DOM;
 import com.google.gwt.user.client.Window;
 import com.google.gwt.user.client.rpc.AsyncCallback;
 import com.google.gwt.user.client.ui.FlowPanel;
@@ -71,7 +69,6 @@ import com.google.gwtexpui.globalkey.client.ShowHelpCommand;
 import net.codemirror.lib.CodeMirror;
 import net.codemirror.lib.CodeMirror.BeforeSelectionChangeHandler;
 import net.codemirror.lib.CodeMirror.GutterClickHandler;
-import net.codemirror.lib.CodeMirror.LineClassWhere;
 import net.codemirror.lib.CodeMirror.LineHandle;
 import net.codemirror.lib.Configuration;
 import net.codemirror.lib.KeyMap;
@@ -120,10 +117,6 @@ public class SideBySide2 extends Screen {
 
   private CodeMirror cmA;
   private CodeMirror cmB;
-  private Element columnMarginA;
-  private Element columnMarginB;
-  private double charWidthPx;
-  private double lineHeightPx;
 
   private HandlerRegistration resizeHandler;
   private ScrollSynchronizer scrollSynchronizer;
@@ -268,12 +261,10 @@ public class SideBySide2 extends Screen {
       }
     });
 
-    final int height = getCodeMirrorHeight();
     operation(new Runnable() {
       @Override
       public void run() {
-        cmA.setHeight(height);
-        cmB.setHeight(height);
+        resizeCodeMirror();
         chunkManager.adjustPadding();
         cmA.refresh();
         cmB.refresh();
@@ -297,6 +288,7 @@ public class SideBySide2 extends Screen {
     if (startSide != null && startLine > 0) {
       int line = startLine - 1;
       CodeMirror cm = getCmFromSide(startSide);
+      int height = cm.getHeight();
       if (cm.lineAtHeight(height - 20) < line) {
         cm.scrollToY(cm.heightAtLine(line, "local") - 0.5 * height);
       }
@@ -549,24 +541,21 @@ public class SideBySide2 extends Screen {
 
   private void display(final CommentsCollections comments) {
     setThemeStyles(prefs.theme().isDark());
-    setShowTabs(prefs.showTabs());
     setShowIntraline(prefs.intralineDifference());
     if (prefs.showLineNumbers()) {
       diffTable.addStyleName(DiffTable.style.showLineNumbers());
     }
 
-    cmA = newCM(diff.meta_a(), diff.text_a(), diffTable.cmA).side(DisplaySide.A);
-    cmB = newCM(diff.meta_b(), diff.text_b(), diffTable.cmB).side(DisplaySide.B);
+    cmA = newCM(diff.meta_a(), diff.text_a(), diffTable.cmA);
+    cmB = newCM(diff.meta_b(), diff.text_b(), diffTable.cmB);
+
+    cmA.extras().side(DisplaySide.A);
+    cmB.extras().side(DisplaySide.B);
+    setShowTabs(prefs.showTabs());
+
     chunkManager = new ChunkManager(this, cmA, cmB, diffTable.scrollbar);
     skipManager = new SkipManager(this, commentManager);
 
-    columnMarginA = DOM.createDiv();
-    columnMarginB = DOM.createDiv();
-    columnMarginA.setClassName(DiffTable.style.columnMargin());
-    columnMarginB.setClassName(DiffTable.style.columnMargin());
-    cmA.mover().appendChild(columnMarginA);
-    cmB.mover().appendChild(columnMarginB);
-
     if (prefs.renderEntireFile() && !canEnableRenderEntireFile(prefs)) {
       // CodeMirror is too slow to layout an entire huge file.
       prefs.renderEntireFile(false);
@@ -679,61 +668,14 @@ public class SideBySide2 extends Screen {
     }
   }
 
-  void setShowTabs(boolean b) {
-    if (b) {
-      diffTable.addStyleName(DiffTable.style.showTabs());
-    } else {
-      diffTable.removeStyleName(DiffTable.style.showTabs());
-    }
+  void setShowTabs(boolean show) {
+    cmA.extras().showTabs(show);
+    cmB.extras().showTabs(show);
   }
 
   void setLineLength(int columns) {
-    double w = columns * getCharWidthPx();
-    columnMarginA.getStyle().setMarginLeft(w, Style.Unit.PX);
-    columnMarginB.getStyle().setMarginLeft(w, Style.Unit.PX);
-  }
-
-  double getLineHeightPx() {
-    if (lineHeightPx <= 1) {
-      Element p = DOM.createDiv();
-      int lines = 1;
-      for (int i = 0; i < lines; i++) {
-        Element e = DOM.createDiv();
-        p.appendChild(e);
-
-        Element pre = DOM.createElement("pre");
-        pre.setInnerText("gqyŚŻŹŃ");
-        e.appendChild(pre);
-      }
-
-      cmB.measure().appendChild(p);
-      lineHeightPx = ((double) p.getOffsetHeight()) / lines;
-      p.removeFromParent();
-    }
-    return lineHeightPx;
-  }
-
-  private double getCharWidthPx() {
-    if (charWidthPx <= 1) {
-      int len = 100;
-      StringBuilder s = new StringBuilder();
-      for (int i = 0; i < len; i++) {
-        s.append('m');
-      }
-      Element e = DOM.createSpan();
-      e.getStyle().setDisplay(Style.Display.INLINE_BLOCK);
-      e.setInnerText(s.toString());
-
-      cmA.measure().appendChild(e);
-      double a = ((double) e.getOffsetWidth()) / len;
-      e.removeFromParent();
-
-      cmB.measure().appendChild(e);
-      double b = ((double) e.getOffsetWidth()) / len;
-      e.removeFromParent();
-      charWidthPx = Math.max(a, b);
-    }
-    return charWidthPx;
+    cmA.extras().lineLength(columns);
+    cmB.extras().lineLength(columns);
   }
 
   void setShowLineNumbers(boolean b) {
@@ -815,15 +757,6 @@ public class SideBySide2 extends Screen {
     return chunkManager.getLineMapper().lineOnOther(side, line);
   }
 
-  private void clearActiveLine(CodeMirror cm) {
-    if (cm.hasActiveLine()) {
-      LineHandle activeLine = cm.activeLine();
-      cm.removeLineClass(activeLine,
-          LineClassWhere.WRAP, DiffTable.style.activeLine());
-      cm.activeLine(null);
-    }
-  }
-
   private Runnable updateActiveLine(final CodeMirror cm) {
     final CodeMirror other = otherCm(cm);
     return new Runnable() {
@@ -842,22 +775,16 @@ public class SideBySide2 extends Screen {
               public void run() {
                 LineHandle handle =
                     cm.getLineHandleVisualStart(cm.getCursor("end").line());
-                if (cm.hasActiveLine() && cm.activeLine().equals(handle)) {
+                if (!cm.extras().activeLine(handle)) {
                   return;
                 }
 
-                clearActiveLine(cm);
-                clearActiveLine(other);
-                cm.activeLine(handle);
-                cm.addLineClass(
-                    handle, LineClassWhere.WRAP, DiffTable.style.activeLine());
                 LineOnOtherInfo info =
                     lineOnOther(cm.side(), cm.getLineNumber(handle));
                 if (info.isAligned()) {
-                  LineHandle oLineHandle = other.getLineHandle(info.getLine());
-                  other.activeLine(oLineHandle);
-                  other.addLineClass(oLineHandle, LineClassWhere.WRAP,
-                      DiffTable.style.activeLine());
+                  other.extras().activeLine(other.getLineHandle(info.getLine()));
+                } else {
+                  other.extras().clearActiveLine();
                 }
               }
             });
@@ -877,8 +804,8 @@ public class SideBySide2 extends Screen {
             && !clickEvent.getAltKey()
             && !clickEvent.getCtrlKey()
             && !clickEvent.getShiftKey()) {
-          if (!(cm.hasActiveLine() &&
-              cm.getLineNumber(cm.activeLine()) == line)) {
+          if (!(cm.extras().hasActiveLine() &&
+              cm.getLineNumber(cm.extras().activeLine()) == line)) {
             cm.setCursor(Pos.create(line));
           }
           Scheduler.get().scheduleDeferred(new ScheduledCommand() {
@@ -928,10 +855,10 @@ public class SideBySide2 extends Screen {
     return new Runnable() {
       @Override
       public void run() {
-        if (cmSrc.hasActiveLine()) {
+        if (cmSrc.extras().hasActiveLine()) {
           cmDst.setCursor(Pos.create(lineOnOther(
               sideSrc,
-              cmSrc.getLineNumber(cmSrc.activeLine())).getLine()));
+              cmSrc.getLineNumber(cmSrc.extras().activeLine())).getLine()));
         }
         cmDst.focus();
       }
@@ -977,17 +904,9 @@ public class SideBySide2 extends Screen {
   }
 
   void resizeCodeMirror() {
-    int height = getCodeMirrorHeight();
-    cmA.setHeight(height);
-    cmB.setHeight(height);
-  }
-
-  private int getCodeMirrorHeight() {
-    int rest = Gerrit.getHeaderFooterHeight()
-        + header.getOffsetHeight()
-        + diffTable.getHeaderHeight()
-        + 5; // Estimate
-    return Window.getClientHeight() - rest;
+    int hdr = header.getOffsetHeight() + diffTable.getHeaderHeight();
+    cmA.adjustHeight(hdr);
+    cmB.adjustHeight(hdr);
   }
 
   void syncScroll(DisplaySide masterSide) {
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.java
new file mode 100644
index 0000000000..fcd03e50af
--- /dev/null
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.java
@@ -0,0 +1,25 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.client.editor;
+
+import com.google.gwt.core.client.GWT;
+import com.google.gwt.i18n.client.Constants;
+
+interface EditConstants extends Constants {
+  static final EditConstants I = GWT.create(EditConstants.class);
+
+  String closeUnsavedChanges();
+  String cancelUnsavedChanges();
+}
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.properties b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.properties
new file mode 100644
index 0000000000..f90d8287f1
--- /dev/null
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditConstants.properties
@@ -0,0 +1,5 @@
+closeUnsavedChanges = Unsaved changes were made to this file.
+
+cancelUnsavedChanges = Unsaved changes were made to this file.\n\
+  \n\
+  Discard unsaved changes?
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.java
index 5c2e5efbeb..70855ce8c7 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.java
@@ -15,6 +15,7 @@
 package com.google.gerrit.client.editor;
 
 import com.google.gerrit.client.Gerrit;
+import com.google.gerrit.client.JumpKeys;
 import com.google.gerrit.client.VoidResult;
 import com.google.gerrit.client.account.DiffPreferences;
 import com.google.gerrit.client.changes.ChangeApi;
@@ -30,13 +31,20 @@ import com.google.gerrit.common.PageLinks;
 import com.google.gerrit.reviewdb.client.Patch;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gwt.core.client.GWT;
+import com.google.gwt.core.client.Scheduler;
+import com.google.gwt.core.client.Scheduler.ScheduledCommand;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.event.dom.client.ClickEvent;
 import com.google.gwt.event.dom.client.KeyPressEvent;
+import com.google.gwt.event.logical.shared.ResizeEvent;
+import com.google.gwt.event.logical.shared.ResizeHandler;
+import com.google.gwt.event.shared.HandlerRegistration;
 import com.google.gwt.uibinder.client.UiBinder;
 import com.google.gwt.uibinder.client.UiField;
 import com.google.gwt.uibinder.client.UiHandler;
 import com.google.gwt.user.client.Window;
+import com.google.gwt.user.client.Window.ClosingEvent;
+import com.google.gwt.user.client.Window.ClosingHandler;
 import com.google.gwt.user.client.rpc.AsyncCallback;
 import com.google.gwt.user.client.ui.Button;
 import com.google.gwt.user.client.ui.HTMLPanel;
@@ -44,9 +52,13 @@ import com.google.gwtexpui.globalkey.client.GlobalKey;
 import com.google.gwtexpui.safehtml.client.SafeHtml;
 
 import net.codemirror.lib.CodeMirror;
+import net.codemirror.lib.CodeMirror.ChangesHandler;
 import net.codemirror.lib.Configuration;
+import net.codemirror.lib.KeyMap;
+import net.codemirror.lib.Pos;
 import net.codemirror.mode.ModeInfo;
 import net.codemirror.mode.ModeInjector;
+import net.codemirror.theme.ThemeLoader;
 
 public class EditScreen extends Screen {
   interface Binder extends UiBinder<HTMLPanel, EditScreen> {}
@@ -58,12 +70,17 @@ public class EditScreen extends Screen {
   private CodeMirror cm;
   private String type;
 
+  @UiField Element header;
   @UiField Element project;
   @UiField Element filePath;
-  @UiField Button cancel;
+  @UiField Button close;
   @UiField Button save;
   @UiField Element editor;
 
+  private HandlerRegistration resizeHandler;
+  private HandlerRegistration closeHandler;
+  private int generation;
+
   public EditScreen(Patch.Key patch) {
     this.revision = patch.getParentKey();
     this.path = patch.get();
@@ -84,9 +101,22 @@ public class EditScreen extends Screen {
     super.onLoad();
 
     CallbackGroup cmGroup = new CallbackGroup();
-    CodeMirror.initLibrary(cmGroup.<Void> addEmpty());
-    CallbackGroup group = new CallbackGroup();
-    if (!Patch.COMMIT_MSG.equals(path)) {
+    final CallbackGroup group = new CallbackGroup();
+    CodeMirror.initLibrary(cmGroup.add(new AsyncCallback<Void>() {
+      final AsyncCallback<Void> themeCallback = group.addEmpty();
+
+      @Override
+      public void onSuccess(Void result) {
+        // Load theme after CM library to ensure theme can override CSS.
+        ThemeLoader.loadTheme(prefs.theme(), themeCallback);
+      }
+
+      @Override
+      public void onFailure(Throwable caught) {
+      }
+    }));
+
+    if (prefs.syntaxHighlighting() && !Patch.COMMIT_MSG.equals(path)) {
       final AsyncCallback<Void> modeInjectorCb = group.addEmpty();
       ChangeFileApi.getContentType(revision, path,
           cmGroup.add(new GerritCallback<String>() {
@@ -121,60 +151,151 @@ public class EditScreen extends Screen {
   @Override
   public void onShowView() {
     super.onShowView();
+    Window.enableScrolling(false);
+    JumpKeys.enable(false);
     if (prefs.hideTopMenu()) {
       Gerrit.setHeaderVisible(false);
     }
-    int rest = Gerrit.getHeaderFooterHeight()
-        + 30; // Estimate
-    cm.setHeight(Window.getClientHeight() - rest);
+    resizeHandler = Window.addResizeHandler(new ResizeHandler() {
+      @Override
+      public void onResize(ResizeEvent event) {
+        cm.adjustHeight(header.getOffsetHeight());
+      }
+    });
+    closeHandler = Window.addWindowClosingHandler(new ClosingHandler() {
+      @Override
+      public void onWindowClosing(ClosingEvent event) {
+        if (!cm.isClean(generation)) {
+          event.setMessage(EditConstants.I.closeUnsavedChanges());
+        }
+      }
+    });
+
+    generation = cm.changeGeneration(true);
+    save.setEnabled(false);
+    cm.on(new ChangesHandler() {
+      @Override
+      public void handle(CodeMirror cm) {
+        save.setEnabled(!cm.isClean(generation));
+      }
+    });
+
+    cm.adjustHeight(header.getOffsetHeight());
+    cm.on("cursorActivity", updateCursorPosition());
+    cm.extras().showTabs(prefs.showTabs());
+    cm.extras().lineLength(prefs.lineLength());
     cm.refresh();
     cm.focus();
+    updateActiveLine();
   }
 
   @Override
   protected void onUnload() {
     super.onUnload();
+    if (cm != null) {
+      cm.getWrapperElement().removeFromParent();
+    }
+    if (resizeHandler != null) {
+      resizeHandler.removeHandler();
+    }
+    if (closeHandler != null) {
+      closeHandler.removeHandler();
+    }
+    Window.enableScrolling(true);
     Gerrit.setHeaderVisible(true);
+    JumpKeys.enable(true);
   }
 
   @UiHandler("save")
   void onSave(@SuppressWarnings("unused") ClickEvent e) {
-    ChangeFileApi.putContentOrMessage(revision, path, cm.getValue(),
-        new GerritCallback<VoidResult>() {
-          @Override
-          public void onSuccess(VoidResult result) {
-            Gerrit.display(PageLinks.toChangeInEditMode(
-                revision.getParentKey()));
-          }
-        });
+    save().run();
   }
 
-  @UiHandler("cancel")
-  void onCancel(@SuppressWarnings("unused") ClickEvent e) {
+  @UiHandler("close")
+  void onClose(@SuppressWarnings("unused") ClickEvent e) {
+    if (cm.isClean(generation)
+        || Window.confirm(EditConstants.I.cancelUnsavedChanges())) {
+      upToChange();
+    }
+  }
+
+  private void upToChange() {
     Gerrit.display(PageLinks.toChangeInEditMode(revision.getParentKey()));
   }
 
   private void initEditor(String content) {
-    cm = CodeMirror.create(editor, getConfig());
-    cm.setValue(content);
+    ModeInfo mode = prefs.syntaxHighlighting()
+        ? ModeInfo.findMode(type, path)
+        : null;
+    cm = CodeMirror.create(editor, Configuration.create()
+        .set("value", content)
+        .set("readOnly", false)
+        .set("cursorBlinkRate", 0)
+        .set("cursorHeight", 0.85)
+        .set("lineNumbers", true)
+        .set("tabSize", prefs.tabSize())
+        .set("lineWrapping", false)
+        .set("scrollbarStyle", "overlay")
+        .set("styleSelectedText", true)
+        .set("showTrailingSpace", true)
+        .set("keyMap", "default")
+        .set("theme", prefs.theme().name().toLowerCase())
+        .set("mode", mode != null ? mode.mode() : null));
+    cm.addKeyMap(KeyMap.create()
+        .on("Cmd-S", save())
+        .on("Ctrl-S", save()));
+  }
+
+  private Runnable updateCursorPosition() {
+    return new Runnable() {
+      @Override
+      public void run() {
+        // The rendering of active lines has to be deferred. Reflow
+        // caused by adding and removing styles chokes Firefox when arrow
+        // key (or j/k) is held down. Performance on Chrome is fine
+        // without the deferral.
+        //
+        Scheduler.get().scheduleDeferred(new ScheduledCommand() {
+          @Override
+          public void execute() {
+            cm.operation(new Runnable() {
+              @Override
+              public void run() {
+                updateActiveLine();
+              }
+            });
+          }
+        });
+      }
+    };
+  }
+
+  private void updateActiveLine() {
+    Pos p = cm.getCursor("end");
+    cm.extras().activeLine(cm.getLineHandleVisualStart(p.line()));
+  }
+
+  private Runnable save() {
+    return new Runnable() {
+      @Override
+      public void run() {
+        if (!cm.isClean(generation)) {
+          String text = cm.getValue();
+          final int g = cm.changeGeneration(false);
+          ChangeFileApi.putContentOrMessage(revision, path, text,
+              new GerritCallback<VoidResult>() {
+                @Override
+                public void onSuccess(VoidResult result) {
+                  generation = g;
+                  save.setEnabled(!cm.isClean(g));
+                }
+              });
+        }
+      }
+    };
   }
 
   private void injectMode(String type, AsyncCallback<Void> cb) {
     new ModeInjector().add(type).inject(cb);
   }
-
-  private Configuration getConfig() {
-    // TODO(davido): Retrieve user preferences from AllUsers repository
-    return Configuration.create()
-        .set("readOnly", false)
-        .set("cursorBlinkRate", 0)
-        .set("cursorHeight", 0.85)
-        .set("lineNumbers", true)
-        .set("tabSize", 4)
-        .set("lineWrapping", false)
-        .set("styleSelectedText", true)
-        .set("showTrailingSpace", true)
-        .set("keyMap", "default")
-        .set("mode", type);
-  }
 }
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.ui.xml b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.ui.xml
index 8033bbf48e..50bd3bc0c8 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.ui.xml
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/editor/EditScreen.ui.xml
@@ -31,7 +31,9 @@ limitations under the License.
     }
 
     .headerButtons button:disabled {
-      background-color: #999;
+      background-color: #ddd;
+      font-weight: normal;
+      cursor: default;
     }
 
     .headerButtons button {
@@ -62,13 +64,13 @@ limitations under the License.
     }
   </ui:style>
   <g:HTMLPanel>
-    <div class='{style.headerLine}'>
+    <div class='{style.headerLine}' ui:field='header'>
        <div class='{style.headerButtons}'>
-         <g:Button ui:field='cancel'
+         <g:Button ui:field='close'
              styleName=''
-             title='Cancel'>
+             title='Close file and return to change'>
            <ui:attribute name='title'/>
-           <div><ui:msg>Cancel</ui:msg></div>
+           <div><ui:msg>Close</ui:msg></div>
          </g:Button>
          <g:Button ui:field='save'
              styleName='{style.save}'
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
index c6e106338c..c731476b93 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/CodeMirror.java
@@ -14,11 +14,14 @@
 
 package net.codemirror.lib;
 
+import com.google.gerrit.client.Gerrit;
 import com.google.gerrit.client.diff.DisplaySide;
 import com.google.gerrit.client.rpc.CallbackGroup;
 import com.google.gwt.core.client.JavaScriptObject;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.NativeEvent;
+import com.google.gwt.resources.client.CssResource;
+import com.google.gwt.user.client.Window;
 import com.google.gwt.user.client.rpc.AsyncCallback;
 
 import net.codemirror.lib.TextMarker.FromTo;
@@ -37,7 +40,23 @@ public class CodeMirror extends JavaScriptObject {
     Loader.initLibrary(cb);
   }
 
-  public static native CodeMirror create(Element p, Configuration cfg) /*-{
+  interface Style extends CssResource {
+    String activeLine();
+    String showTabs();
+    String margin();
+  }
+
+  static Style style() {
+    return Lib.I.style();
+  }
+
+  public static CodeMirror create(Element p, Configuration cfg) {
+    CodeMirror cm = newCM(p, cfg);
+    Extras.attach(cm);
+    return cm;
+  }
+
+  private static native CodeMirror newCM(Element p, Configuration cfg) /*-{
     return $wnd.CodeMirror(p, cfg);
   }-*/;
 
@@ -62,10 +81,25 @@ public class CodeMirror extends JavaScriptObject {
   public final native String getValue() /*-{ return this.getValue() }-*/;
   public final native void setValue(String v) /*-{ this.setValue(v) }-*/;
 
+  public final native int changeGeneration(boolean closeEvent)
+  /*-{ return this.changeGeneration(closeEvent) }-*/;
+  public final native boolean isClean(int generation)
+  /*-{ return this.isClean(generation) }-*/;
+
   public final native void setWidth(double w) /*-{ this.setSize(w, null) }-*/;
-  public final native void setWidth(String w) /*-{ this.setSize(w, null) }-*/;
   public final native void setHeight(double h) /*-{ this.setSize(null, h) }-*/;
-  public final native void setHeight(String h) /*-{ this.setSize(null, h) }-*/;
+
+  public final int getHeight() {
+    return getWrapperElement().getClientHeight();
+  }
+
+  public final void adjustHeight(int localHeader) {
+    int rest = Gerrit.getHeaderFooterHeight()
+        + localHeader
+        + 5; // Estimate
+    setHeight(Window.getClientHeight() - rest);
+  }
+
   public final native String getLine(int n) /*-{ return this.getLine(n) }-*/;
   public final native double barHeight() /*-{ return this.display.barHeight }-*/;
   public final native double barWidth() /*-{ return this.display.barWidth }-*/;
@@ -226,6 +260,13 @@ public class CodeMirror extends JavaScriptObject {
     }))
   }-*/;
 
+  public final native void on(ChangesHandler handler) /*-{
+    this.on('changes', $entry(function(cm, o) {
+      handler.@net.codemirror.lib.CodeMirror.ChangesHandler::handle(
+        Lnet/codemirror/lib/CodeMirror;)(cm);
+    }))
+  }-*/;
+
   public final native void setCursor(Pos p) /*-{ this.setCursor(p) }-*/;
   public final native Pos getCursor() /*-{ return this.getCursor() }-*/;
   public final native Pos getCursor(String start) /*-{
@@ -245,18 +286,6 @@ public class CodeMirror extends JavaScriptObject {
     return this.somethingSelected()
   }-*/;
 
-  public final native boolean hasActiveLine() /*-{
-    return !!this.state.activeLine
-  }-*/;
-
-  public final native LineHandle activeLine() /*-{
-    return this.state.activeLine
-  }-*/;
-
-  public final native void activeLine(LineHandle line) /*-{
-    this.state.activeLine = line
-  }-*/;
-
   public final native void addKeyMap(KeyMap map) /*-{ this.addKeyMap(map) }-*/;
   public final native void removeKeyMap(KeyMap map) /*-{ this.removeKeyMap(map) }-*/;
 
@@ -321,11 +350,13 @@ public class CodeMirror extends JavaScriptObject {
     return this;
   }-*/;
 
-  public final native DisplaySide side() /*-{ return this._sbs2_side }-*/;
-  public final native CodeMirror side(DisplaySide side) /*-{
-    this._sbs2_side = side;
-    return this;
-  }-*/;
+  public final DisplaySide side() {
+    return extras().side();
+  }
+
+  public final Extras extras() {
+    return Extras.get(this);
+  }
 
   protected CodeMirror() {
   }
@@ -367,4 +398,8 @@ public class CodeMirror extends JavaScriptObject {
   public interface BeforeSelectionChangeHandler {
     public void handle(CodeMirror instance, Pos anchor, Pos head);
   }
+
+  public interface ChangesHandler {
+    public void handle(CodeMirror instance);
+  }
 }
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/Extras.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/Extras.java
new file mode 100644
index 0000000000..13186d19a2
--- /dev/null
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/Extras.java
@@ -0,0 +1,143 @@
+// Copyright (C) 2015 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net.codemirror.lib;
+
+import static com.google.gwt.dom.client.Style.Display.INLINE_BLOCK;
+import static com.google.gwt.dom.client.Style.Unit.PX;
+import static net.codemirror.lib.CodeMirror.style;
+import static net.codemirror.lib.CodeMirror.LineClassWhere.WRAP;
+
+import com.google.gerrit.client.diff.DisplaySide;
+import com.google.gwt.dom.client.Element;
+import com.google.gwt.user.client.DOM;
+
+import net.codemirror.lib.CodeMirror.LineHandle;
+
+import java.util.Objects;
+
+/** Additional features added to CodeMirror by Gerrit Code Review. */
+public class Extras {
+  static final native Extras get(CodeMirror c) /*-{ return c.gerritExtras }-*/;
+  private static final native void set(CodeMirror c, Extras e)
+  /*-{ c.gerritExtras = e }-*/;
+
+  static void attach(CodeMirror c) {
+    set(c, new Extras(c));
+  }
+
+  private final CodeMirror cm;
+  private Element margin;
+  private DisplaySide side;
+  private double charWidthPx;
+  private double lineHeightPx;
+  private LineHandle activeLine;
+
+  private Extras(CodeMirror cm) {
+    this.cm = cm;
+  }
+
+  public DisplaySide side() {
+    return side;
+  }
+
+  public void side(DisplaySide s) {
+    side = s;
+  }
+
+  public double charWidthPx() {
+    if (charWidthPx <= 1) {
+      int len = 100;
+      StringBuilder s = new StringBuilder();
+      for (int i = 0; i < len; i++) {
+        s.append('m');
+      }
+
+      Element e = DOM.createSpan();
+      e.getStyle().setDisplay(INLINE_BLOCK);
+      e.setInnerText(s.toString());
+
+      cm.measure().appendChild(e);
+      charWidthPx = ((double) e.getOffsetWidth()) / len;
+      e.removeFromParent();
+    }
+    return charWidthPx;
+  }
+
+  public double lineHeightPx() {
+    if (lineHeightPx <= 1) {
+      Element p = DOM.createDiv();
+      int lines = 1;
+      for (int i = 0; i < lines; i++) {
+        Element e = DOM.createDiv();
+        p.appendChild(e);
+
+        Element pre = DOM.createElement("pre");
+        pre.setInnerText("gqyŚŻŹŃ");
+        e.appendChild(pre);
+      }
+
+      cm.measure().appendChild(p);
+      lineHeightPx = ((double) p.getOffsetHeight()) / lines;
+      p.removeFromParent();
+    }
+    return lineHeightPx;
+  }
+
+  public void lineLength(int columns) {
+    if (margin == null) {
+      margin = DOM.createDiv();
+      margin.setClassName(style().margin());
+      cm.mover().appendChild(margin);
+    }
+    margin.getStyle().setMarginLeft(columns * charWidthPx(), PX);
+  }
+
+  public void showTabs(boolean show) {
+    Element e = cm.getWrapperElement();
+    if (show) {
+      e.addClassName(style().showTabs());
+    } else {
+      e.removeClassName(style().showTabs());
+    }
+  }
+
+  public final boolean hasActiveLine() {
+    return activeLine != null;
+  }
+
+  public final LineHandle activeLine() {
+    return activeLine;
+  }
+
+  public final boolean activeLine(LineHandle line) {
+    if (Objects.equals(activeLine, line)) {
+      return false;
+    }
+
+    if (activeLine != null) {
+      cm.removeLineClass(activeLine, WRAP, style().activeLine());
+    }
+    activeLine = line;
+    cm.addLineClass(activeLine, WRAP, style().activeLine());
+    return true;
+  }
+
+  public final void clearActiveLine() {
+    if (activeLine != null) {
+      cm.removeLineClass(activeLine, WRAP, style().activeLine());
+      activeLine = null;
+    }
+  }
+}
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/Lib.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/Lib.java
index 874d1867f6..a3b1cf453f 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/lib/Lib.java
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/Lib.java
@@ -29,4 +29,7 @@ interface Lib extends ClientBundle {
   @Source("cm.js")
   @DoNotEmbed
   DataResource js();
+
+  @Source("style.css")
+  CodeMirror.Style style();
 }
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/Loader.java b/gerrit-gwtui/src/main/java/net/codemirror/lib/Loader.java
index 2643f846d0..f2822b7c6a 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/lib/Loader.java
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/Loader.java
@@ -56,6 +56,7 @@ public class Loader {
         @Override
         public void onSuccess(TextResource resource) {
           StyleInjector.inject(resource.getText());
+          Lib.I.style().ensureInjected();
           cb.onSuccess(null);
         }
 
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/lib/style.css b/gerrit-gwtui/src/main/java/net/codemirror/lib/style.css
new file mode 100644
index 0000000000..c5f79d3c56
--- /dev/null
+++ b/gerrit-gwtui/src/main/java/net/codemirror/lib/style.css
@@ -0,0 +1,83 @@
+/* Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+@external .CodeMirror;
+@external .CodeMirror-lines;
+@external .CodeMirror-linenumber;
+@external .CodeMirror-overlayscroll-horizontal;
+@external .CodeMirror-overlayscroll-vertical;
+@external .cm-tab;
+@external .cm-searching;
+@external .cm-trailingspace;
+
+/* Reduce margins around CodeMirror to save space. */
+.CodeMirror-lines {
+  padding: 0;
+}
+.CodeMirror pre {
+  padding: 0;
+  line-height: normal;
+}
+
+/* Minimum scrollbar bubble size even on large files. */
+.CodeMirror-overlayscroll-horizontal div {
+  min-width: 25px;
+}
+.CodeMirror-overlayscroll-vertical div {
+  min-height: 25px;
+}
+
+/* Stack the scrollbar so annotations can receive clicks. */
+.CodeMirror-overlayscroll-vertical {
+  z-index: inherit;
+}
+.CodeMirror-overlayscroll-horizontal div,
+.CodeMirror-overlayscroll-vertical div {
+  background-color: rgba(128, 128, 128, 0.50);
+  z-index: 8;
+}
+
+/* Highlight current line number in the line gutter. */
+.activeLine .CodeMirror-linenumber {
+  background-color: #bcf !important;
+  color: #000;
+}
+
+.showTabs .cm-tab:before {
+  position: absolute;
+  content: "\00bb";
+  color: #f00;
+}
+
+.cm-searching {
+  background-color: #ffa;
+}
+
+.cm-trailingspace {
+  background-color: red;
+}
+
+/* Line length margin displayed at NN columns to provide
+ * a visual guide for length of any single line of code.
+ */
+.margin {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  width: 0;
+  border-right: 1px dashed #ffa500;
+  z-index: 2;
+  cursor: text;
+}